U.S. patent application number 14/901147 was filed with the patent office on 2016-05-26 for steel sheet for hot press-forming (as amended).
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Satoru Ando, Minako Morimoto, Seiji Nakajima.
Application Number | 20160144600 14/901147 |
Document ID | / |
Family ID | 52141359 |
Filed Date | 2016-05-26 |
United States Patent
Application |
20160144600 |
Kind Code |
A1 |
Nakajima; Seiji ; et
al. |
May 26, 2016 |
STEEL SHEET FOR HOT PRESS-FORMING (AS AMENDED)
Abstract
A steel sheet for hot press-forming having a first coating layer
on a surface of the steel sheet, containing 60% or more by mass Ni
and a remainder composed of Zn and incidental impurities, a coating
mass thereof being more than 5 g/m.sup.2 and 50 g/m.sup.2 or less
per side, and a second coating layer on the first coating layer,
containing 10% to 25% by mass Ni and a remainder composed of Zn and
incidental impurities, a coating mass thereof being 10 to 90
g/m.sup.2 per side.
Inventors: |
Nakajima; Seiji; (Tokyo,
JP) ; Morimoto; Minako; (Tokyo, JP) ; Ando;
Satoru; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
52141359 |
Appl. No.: |
14/901147 |
Filed: |
May 13, 2014 |
PCT Filed: |
May 13, 2014 |
PCT NO: |
PCT/JP2014/002505 |
371 Date: |
December 28, 2015 |
Current U.S.
Class: |
428/656 |
Current CPC
Class: |
C22C 38/60 20130101;
C22C 19/03 20130101; C25D 3/56 20130101; C25D 5/14 20130101; C25D
3/12 20130101; Y10T 428/12799 20150115; C22C 38/001 20130101; C25D
7/0614 20130101; C22C 38/32 20130101; B32B 15/015 20130101; C21D
1/00 20130101; C21D 1/673 20130101; C25D 5/50 20130101; C22C 38/28
20130101; C22C 38/02 20130101; C22C 38/06 20130101; C21D 7/13
20130101; Y10T 428/12937 20150115; C22C 18/00 20130101; Y10T
428/12944 20150115; C22C 38/04 20130101; C25D 3/565 20130101; C22C
38/002 20130101 |
International
Class: |
B32B 15/01 20060101
B32B015/01; C22C 18/00 20060101 C22C018/00; C22C 38/60 20060101
C22C038/60; C22C 38/00 20060101 C22C038/00; C22C 38/28 20060101
C22C038/28; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 19/03 20060101
C22C019/03; C22C 38/32 20060101 C22C038/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 25, 2013 |
JP |
2013-132248 |
Claims
1. A steel sheet for hot press-forming comprising: a first coating
layer on a surface of the steel sheet, containing 60% or more by
mass Ni and a remainder composed of Zn and incidental impurities, a
coating mass thereof being more than 5 g/m.sup.2 and 50 g/m.sup.2
or less per side; and a second coating layer on the first coating
layer, containing 10% to 25% by mass Ni and a remainder composed of
Zn and incidental impurities, a coating mass thereof being 10 to 90
g/m.sup.2 per side.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is the U.S. National Phase application of
PCT/JP2014/002505, filed May 13, 2014, which claims priority to
Japanese Patent Application No. 2013-132248, filed Jun. 25, 2013,
the disclosures of these applications being incorporated herein by
reference in their entireties for all purposes.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a steel sheet for hot
press-forming that is suitable to produce automotive underbody
parts and automotive body structural parts by hot
press-forming.
BACKGROUND OF THE INVENTION
[0003] Most of the automotive underbody parts and the automotive
body structural parts are manufactured by press-forming steel
sheets having a predetermined strength at a room temperature. In
recent years, from the perspective of global environmental
conservation, there has been a strong demand for weight reduction
of automotive bodies, and efforts have been made to reinforce steel
sheets and thereby decrease the thickness of the steel sheets.
However, the reinforced steel sheets have lower press-formability,
and it is often difficult to press-form the steel sheets into
automotive parts having desired shapes.
[0004] A technique, called a hot press-forming, is proposed in
Patent Literature 1, in which both good press-formability and
reinforcement of steel sheets are satisfied by hot press-forming a
heated steel sheet with a mold composed of a die and a punch and
simultaneously quenching the heated steel sheet. However, in the
hot press-forming, the steel sheet is heated to a high temperature
of approximately 950.degree. C. before the hot press-forming, and
scales (iron oxides) are formed on the surface of the steel sheet.
Such scales are peeled off during the hot press-forming and causes
a problem of damage to the mold or damage to the surface of hot
press-formed parts.
[0005] Scales remaining on the surface of the parts also causes
poor appearance and poor paint adhesiveness. Thus, the scales on
the surface of the parts are generally removed by a treatment such
as pickling or shot blasting. However, such a treatment makes the
production process complex and decreases productivity.
[0006] Automotive underbody parts and automotive body structural
parts also require excellent corrosion resistance. However, hot
press-formed parts manufactured through such a treatment have no
anticorrosion film, such as a coating layer, and have quite
insufficient corrosion resistance.
[0007] Thus, there is a demand for a hot press-forming technique
that can suppress the formation of scales during heating before hot
press-forming and improve the corrosion resistance of hot
press-formed parts. Therefore, a steel sheet for hot press-forming
that has a film, such as a coating layer, on its surface and a hot
press-forming method in which such a steel sheet is used are
proposed. For example, Patent Literature 2 discloses a method for
producing a hot press-formed part excellent in corrosion resistance
having Zn--Fe-based or Zn--Fe--Al-based compounds on its surface by
hot press-forming a steel sheet coated with Zn or Zn-based
alloy.
[0008] Patent Literature 3 discloses a steel sheet for hot
press-forming that includes a Ni-based coating layer and a Zn--Ni
coating layer in this order on its surface. The Ni-based coating
layer has a coating mass of 0.01 to 5 g/m.sup.2, and the Zn--Ni
coating layer has a coating mass of 10 to 90 g/m.sup.2.
CITATION LIST
Patent Literature
[0009] PTL 1: British Patent No. 1490535
[0010] PTL 2: Japanese Patent No. 3663145
[0011] PTL 3: Japanese Patent No. 4883240
SUMMARY OF THE INVENTION
[0012] However, a galvanized steel sheet or a zinc aluminum coated
steel sheet having a coating layer of low melting point is used in
hot press-formed parts manufactured by the method described in
Patent Literature 2. This can cause liquid-metal brittle fracture
during hot press-forming, in which zinc in the coating layer enters
the steel sheet and causes cracks.
[0013] In recent years, there has been a strong demand for
improving productivity due to a reduction in hot press-forming tact
time. Thus, in various steps of hot press-forming, studies have
been made to quickly transfer a heated steel sheet to a
press-forming machine and immediately hot press-form the steel
sheet, that is, to decrease a transfer time. However, when the
steel sheet for hot press-forming described in Patent Literature 3
is subjected to hot press-forming after a short transfer time,
liquid-metal brittle fracture can be caused. Thus, in order to
prevent the fracture, the start of hot press-forming is
intentionally delayed. This deteriorates productivity.
[0014] In view of the situations described above, aspects of the
present invention provide a steel sheet for hot press-forming
having excellent liquid-metal brittle resistance that ensures no
liquid-metal brittle fracture even under hot press-forming
conditions including a short transfer time so as to improve
productivity.
[0015] The present inventors extensively studied steel sheets for
hot press-forming in order to solve the problems described above.
As a result, the present inventors found that in order to attain
the advantages associated with aspects of the present invention the
following steel sheet for hot press-forming is effective and
preferable. That is, the steel sheet for hot press-forming has two
coating layers on its surface. The first coating layer having a
high melting point contains 60% or more by mass Ni and a remainder
composed of Zn and incidental impurities, a coating mass thereof
being more than 5 g/m.sup.2 and 50 g/m.sup.2 or less. The second
coating layer on the first coating layer contains 10% to 25% by
mass Ni and a remainder composed of Zn and incidental impurities, a
coating mass thereof being 10 to 90 g/m.sup.2.
[0016] Aspects of the present invention are based on such findings
and provides a steel sheet for hot press-forming, characterized by
having;
the first coating layer on the surface of the steel sheet,
containing 60% or more by mass Ni and a remainder composed of Zn
and incidental impurities, a coating mass thereof being more than 5
g/m.sup.2 and 50 g/m.sup.2 or less per side, and the second coating
layer on the first coating layer, containing 10% to 25% by mass Ni
and a remainder composed of Zn and incidental impurities, a coating
mass thereof being 10 to 90 g/m.sup.2 per side.
[0017] A steel sheet for hot press-forming according to aspects of
the present invention can be used to manufacture hot press-formed
parts having excellent liquid-metal brittle resistance that ensures
no liquid-metal brittle fracture even under hot press-forming
conditions including a short transfer time so as to improve
productivity. Hot press-formed parts manufactured from the steel
sheet for hot press-forming according to aspects of the present
invention have excellent liquid-metal brittle resistance and are
suitable for automotive underbody parts and automotive body
structural parts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a drawing of hat-shaped part manufactured by hot
press-forming a steel sheet in Example 1.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
1) Coating Layer
[0019] One aspects of the present invention is characterized by
having the first coating layer on the surface of the steel sheet
containing 60% or more by mass Ni and a remainder composed of Zn
and incidental impurities, a coating weight of more than 5
g/m.sup.2 and 50 g/m.sup.2 or less per side, and the second coating
layer on the first coating layer, containing 10% to 25% by mass Ni
and a remainder composed of Zn and incidental impurities, a coating
mass thereof being 10 to 90 g/m.sup.2 per side, in order to ensure
excellent liquid-metal brittle resistance even under hot
press-forming conditions including a short transfer time so as to
improve productivity.
[0020] A short transfer time results in a high initial temperature
of hot press-forming. Thus, the hot press-forming may be started
before the coating layer is completely solidified. This sometimes
causes liquid-metal brittle fracture. In accordance with aspects of
the present invention, the first coating layer on the surface of
the steel sheet can completely prevent zinc in the second coating
layer described below from entering the steel sheet. The first
coating layer contains 60% or more by mass Ni and a remainder
composed of Zn and incidental impurities, a coating mass thereof
being more than 5 g/m.sup.2 and 50 g/m.sup.2 or less per side. This
can ensure excellent liquid-metal brittle resistance even under hot
press-forming conditions including a short transfer time.
[0021] The reason that the Ni content of the first coating layer is
60% or more by mass is that the first coating layer can have a very
high melting point of 1000.degree. C. or more so as not to melt
during heating before hot press-forming.
[0022] The first coating layer has a coating mass of more than 5
g/m.sup.2 and 50 g/m.sup.2 or less per side. The first coating
layer with a coating mass of 5 g/m.sup.2 or less has no shielding
effect so as to prevent zinc in the second coating layer from
entering the steel sheet. The shielding effect levels off at a
coating mass of 50 g/m.sup.2, and the coating mass of more than 50
g/m.sup.2 results in increased costs. Thus, the first coating layer
has a coating mass of more than 5 g/m.sup.2 and 50 g/m.sup.2 or
less.
[0023] In accordance with aspects of the present invention, the
second coating layer is disposed on the first coating layer. The
second coating layer contains 10% to 25% by mass Ni and a remainder
composed of Zn and incidental impurities and has a coating mass of
10 to 90 g/m.sup.2 per side.
[0024] The reason that the Ni content of the second coating layer
ranges from 10% to 25% by mass is that the phase structure of the
second coating layer can be a corrosion resistant .gamma. phase
having a melting point of 881.degree. C. When the coating layer has
Ni content in the range of 10% to 25% by mass, the .gamma. phase
having a crystal structure of Ni.sub.2Zn.sub.11, NiZn.sub.3, or
Ni.sub.5Zn.sub.21 and a melting point of 881.degree. C. is formed,
thereby minimizing ZnO formation reaction during heating. After hot
press-forming, the coating layer remains as the .gamma. phase and
has high corrosion resistance due to the sacrificial protection
effect of Zn. The formation of the .gamma. phase at Ni content in
the range of 10% to 25% by mass does not necessarily conform to the
equilibrium diagram of Ni--Zn alloy. This is probably because the
coating layer formation reaction by an electroplating method or the
like proceeds in a non-equilibrium manner. The .gamma. phase has a
crystal structure of Ni.sub.2Zn.sub.11, NiZn.sub.3, or
Ni.sub.5Zn.sub.21 and can be identified by an X-ray diffraction
method or an electron diffraction method utilizing a transmission
electron microscope (TEM).
[0025] In accordance with aspects of the present invention, the
second coating layer has a coating mass of 10 to 90 g/m.sup.2 per
side. The coating mass of less than 10 g/m.sup.2 results in hot
press-formed parts having insufficient corrosion resistance. The
coating mass of more than 90 g/m.sup.2 results in increased costs.
Thus, the second coating layer should be at a coating mass of 10 to
90 g/m.sup.2.
[0026] The coating weight of coating layer can be determined by a
wet analysis method. More specifically, for example, a coating
layer having a known deposition area is dissolved in 6% by mass
aqueous hydrochloric acid to which 1 g/l of hexamethylenetetramine
is added as an inhibitor, and the coating weight of coating layer
can be determined from the weight loss.
[0027] A method for forming such a coating layer is preferably, but
not limited to, a known electroplating method. The coating weight
of coating layer can be controlled by adjusting the energization
time.
2) Base Steel Sheet
[0028] In order to manufacture hot press-formed parts having a
tensile strength of 980 MPa or more, the base steel sheet may be a
hot-rolled steel sheet or a cold-rolled steel sheet having a
composition consisting of C: 0.15% to 0.50%, Si: 0.05% to 2.00%,
Mn: 0.5% to 3.0%, P: 0.10% or less, S: 0.05% or less, Al: 0.10% or
less, N: 0.010% or less on a mass percent basis, and the remainder
being Fe and incidental impurities. The reasons for limiting the
content of each element will be described below. Unless otherwise
specified, "%" of the component content refers to "% by mass".
C: 0.15 to 0.50%
[0029] C is an element for improving strength of steel. In order
for hot press-formed parts to have a tensile strength (TS) of 980
MPa or more, the C content should be 0.15% or more. On the other
hand, the C content of more than 0.50% results in very low blanking
formability of the base steel sheet. Thus, the C content ranges
from 0.15% to 0.50%.
Si: 0.05% to 2.00%
[0030] Like C, Si is an element for improving strength of steel. In
order for hot press-formed parts to have a TS of 980 MPa or more,
the Si content should be 0.05% or more. On the other hand, the Si
content of more than 2.00% results in significantly increased
surface defects called red scales during hot-rolling, increased
hot-rolling load, and low ductility of hot-rolled steel sheets.
Furthermore, the Si content of more than 2.00% may adversely affect
plating treatability in plating treatment of forming a coating
layer composed mainly of Zn or Al on the surface of the base steel
sheet. Thus, the Si content ranges from 0.05% to 2.00%.
Mn: 0.5% to 3.0%
[0031] Mn is an element effective in suppressing ferrite
transformation and improving hardenability. Mn decreases the
Ac.sub.3 transformation point and is therefore an element effective
in decreasing the heating temperature before hot press-forming.
These effects require a Mn content of 0.5% or more. On the other
hand, the Mn content of more than 3.0% results in a lack of
characteristic uniformity of the base steel sheet and hot
press-formed parts due to its segregation. Thus, the Mn content
ranges from 0.5% to 3.0%.
P: 0.10% or Less
[0032] The P content of more than 0.10% results in a lack of
characteristic uniformity and a significant decrease in toughness
of the base steel sheet and hot press-formed parts due to its
segregation. Thus, the P content is 0.10% or less.
S: 0.05% or Less
[0033] The S content of more than 0.05% results in a decrease in
toughness of hot press-formed parts. Thus, the S content is 0.05%
or less.
Al: 0.10% or Less
[0034] The Al content of more than 0.10% results in low blanking
formability or low hardenability of the base steel sheet. Thus, the
Al content is 0.10% or less.
N: 0.010% or Less
[0035] The N content of more than 0.010% results in formation of
nitrides AlN during hot-rolling or during heating before hot
press-forming, thus resulting in low blanking formability or low
hardenability of the base steel sheet. Thus, the N content is
0.010% or less.
[0036] The remainder is Fe and incidental impurities. For the
following reasons, at least one selected from Cr: 0.01% to 1.0%,
Ti: 0.20% or less, and B: 0.0005% to 0.0800%, and/or Sb: 0.003% to
0.030% is preferably individually or simultaneously contained.
Cr: 0.01% to 1.0%
[0037] Cr is an element effective in improving strength and
hardenability of steel. The effect preferably requires a Cr content
of 0.01% or more. On the other hand, the Cr content of more than
1.0% results in significantly increased costs. Thus, the upper
limit of the Cr content is preferably 1.0%.
Ti: 0.20% or Less
[0038] Ti is an element effective in improving strength and
toughness of steel due to a decrease in grain size. Ti is also an
element that forms nitrides in preference to B described below and
is effective in improving hardenability due to solute B. However,
the Ti content of more than 0.20% results in extremely increased
hot-rolling load and low toughness of hot press-formed parts. Thus,
the upper limit of the Ti content is preferably 0.20%.
B: 0.0005% to 0.0800%
[0039] B is an element effective in improving hardenability and
toughness after hot press-forming. The effect preferably requires a
B content of 0.0005% or more. On the other hand, the B content of
more than 0.0800% results in extremely increased hot-rolling load
and a fracture of hot-rolled steel sheet due to the formation of
martensite phase or bainite phase. Thus, the upper limit of the B
content is preferably 0.0800%.
Sb: 0.003% to 0.030%
[0040] Sb has an effect of suppressing the formation of
decarburized layer in a steel sheet surface layer in a hot
press-forming process from heating before hot press-forming to
cooling of the steel sheet. The effect requires a Sb content of
0.003% or more. On the other hand, the Sb content of more than
0.030% results in increased rolling load and low productivity.
Thus, the Sb content preferably ranges from 0.003% to 0.030%.
3) Method for Manufacturing Hot Press-Formed Parts
[0041] The steel sheet for hot press-forming according to aspects
of the present invention can be hot press-formed using common
procedures to manufacture hot press-formed parts. For example, the
steel sheet for hot press-forming is heated to a temperature in the
range of 850.degree. C. to 950.degree. C., which is higher than or
equal to the Ac.sub.3 transformation point, and then subjected to
hot press-forming. As a method for heating the steel sheet before
hot press-forming, heating with an electric furnace or a gas
furnace, flame heating, electrical resistance heating,
high-frequency heating, induction heating, or far-infrared heating
may be applied.
[0042] The steel sheet for hot press-forming thus heated is placed
in a mold composed of a die and a punch, subjected to hot
press-forming, and cooled under desired cooling conditions, thus
manufacturing hot press-formed parts.
EXAMPLE 1
[0043] A cold-rolled steel sheet having a thickness of 1.6 mm was
used as a base steel sheet. The cold-rolled steel sheet had a
composition consisting of C: 0.23%, Si: 0.25%, Mn: 1.2%, P: 0.01%,
S: 0.01%, Al: 0.03%, N: 0.005%, Cr: 0.2%, Ti: 0.02%, B: 0.0022%,
Sb: 0.008% on a mass percent basis, and the remainder being Fe and
incidental impurities, and had an Ac.sub.3 transformation point of
820.degree. C. The first coating layer and the second coating layer
were formed in this order on both sides of the cold-rolled steel
sheet by electroplating. Thus, steel sheets Nos. 1 to 16 were
manufactured.
[0044] The first coating layer was formed in a plating bath
containing 240 g/L nickel sulfate hexahydrate and 30 g/L boric acid
at a pH of 3.6, at a bath temperature of 50.degree. C., and at a
current density of 5 A/dm.sup.2. The coating weight of coating
layer was adjusted by changing the energization time. For the steel
sheets Nos. 2 and 11, the Ni content was adjusted by the addition
of zinc sulfate heptahydrate. The second coating layer was formed
in a plating bath containing 200 g/L nickel sulfate hexahydrate and
10 to 100 g/L zinc sulfate heptahydrate at a pH of 1.5, at a bath
temperature of 50.degree. C., and at a current density in the range
of 5 to 100 A/dm.sup.2. The Ni content was adjusted by changing the
amount of zinc sulfate heptahydrate and the current density. The
coating weight of coating layer was adjusted by changing the
energization time.
[0045] 100 mm.times.200 mm test specimens were taken from the steel
sheets Nos. 1 to 16 thus manufactured. The steel sheets were heated
to the heating temperature listed in Table 1 in an electric
furnace. Hot press-forming was then performed at an initial forming
temperature listed in Table 1. A hat-shaped mold composed of a
punch having a shoulder R 6 mm in radius and a die having a
shoulder R 6 mm in radius was used in the hot press-forming. A
hat-shaped part having a height of 40 mm illustrated in FIG. 1 was
manufactured by hat-shaped forming at a blank holding pressure of
20 ton and at a speed of 200 mm/s.
[0046] In general hot press-forming, the cooling rate of steel
sheet in a transfer step from heating to press-forming is
approximately 20.degree. C./s. For example, when a steel sheet
heated to 900.degree. C. is transferred to a pressing machine at a
transfer time of approximately 10 seconds, the initial forming
temperature is approximately 700.degree. C. In the present example,
the transfer time was controlled from 4 to 6 seconds, which is
shorter than the general transfer time, and the initial forming
temperature was as high as 820.degree. C. or more.
[0047] Hat-shaped parts thus manufactured were evaluated for
liquid-metal brittle resistance by the following method.
[0048] Liquid-metal brittle resistance: A sample for
cross-sectional observation was taken from the shoulder R (an outer
surface side) of hat-shaped part, and observed with a scanning
electron microscope (SEM). The maximum crack depth observed in the
base steel sheet was evaluated as follows.
[0049] Circle (.largecircle.): Maximum crack depth=0 mm (no
cracks)
[0050] Cross (X): Maximum crack depth .gtoreq.0.01 mm (cracks
occurred)
[0051] Table 1 lists the details of the coating layers, the hot
press-forming conditions, and the liquid-metal brittle resistance
evaluation results of the steel sheets Nos. 1 to 16.
TABLE-US-00001 TABLE 1 First coating Second coating Hot
press-forming Liquid-metal brittle layer layer conditions
resistance Steel Coating Coating Heating Initial forming Maximum
sheet Ni content mass* Ni content mass* temperature temperature
crack depth No. (mass %) (g/m2) (mass %) (g/m2) (.degree. C.)
(.degree. C.) (mm) Evaluation Remarks 1 100 10 12 50 900 820 0
.smallcircle. Invention Example 2 60 10 12 50 900 820 0
.smallcircle. Invention Example 3 100 6 12 50 900 820 0
.smallcircle. Invention Example 4 100 50 12 50 900 820 0
.smallcircle. Invention Example 5 100 10 10 50 900 820 0
.smallcircle. Invention Example 6 100 10 25 50 900 820 0
.smallcircle. Invention Example 7 100 10 12 10 900 820 0
.smallcircle. Invention Example 8 100 10 12 90 900 820 0
.smallcircle. Invention Example 9 100 10 12 50 950 860 0
.smallcircle. Invention Example 10 100 10 12 50 1000 890 0
.smallcircle. Invention Example 11 50 10 12 50 900 820 0.1 x
Comparative example 12 100 0.1 12 50 900 820 0.15 x Comparative
example 13 100 1 12 50 900 820 0.1 x Comparative example 14 100 4
12 50 900 820 0.05 x Comparative example 15 100 4 12 50 950 860 0.2
x Comparative example 16 100 4 12 50 1000 890 0.3 x Comparative
example *per side
[0052] The steel sheets for hot press-forming Nos. 1 to 10
according to aspects of the present invention have excellent
liquid-metal brittle resistance even under the hot press-forming
conditions including a high initial forming temperature, that is,
under the hot press-forming conditions including a short transfer
time.
[0053] All of the hat-shaped parts manufactured using the steel
sheets Nos. 1 to 10 according to aspects of the present invention
and the steel sheets Nos. 11 to 16 according to the comparative
examples had a tensile strength of 980 MPa or more.
* * * * *